1. Field of the Invention
The present invention relates to a surface-coated cermet cutting tool (hereinafter, referred to as a coated cermet tool) in which a hard coating layer exhibits excellent chipping resistance, in particular, even when intermittent cutting of various workpieces, such as steel and cast iron, is performed under high-speed cutting conditions.
2. Description of the Related Art
A coated cermet tool generally formed by depositing on a surface of a substrate (hereinafter, generally referred to as a tool substrate) made of tungsten carbide (hereinafter, referred to as WC)—based cemented carbide or titanium carbonitride (hereinafter, referred to as TiCN)—based cermet a hard-coating layer composed of the following upper and lower layers:
(a) the lower layer is composed of one or more layers of titanium carbide (hereinafter, referred to as TiC) layer, titanium nitride (hereinafter, referred to as TiN) layer, titanium carbonitride (hereinafter, referred to as TiCN) layer, titanium carboxide (hereinafter, referred to as TiCO) layer, and titanium oxycarbonitride (hereinafter, referred to as TiCNO) layer and has a total average layer thickness in the range of 3 to 20 μm, and
(b) the upper layer is an aluminum oxide layer having an α-crystal structure (hereinafter, referred to as an α-Al2O3 layer) when chemically deposited and has an average layer thickness in the range of 1 to 15 μm
is known, JP-A-6-31503 and it is also well known that the coated cermet tool is widely used for continuous or intermittent cutting of, for example, various steel or cast iron.
In addition, it is well known JP-A-6-8010 that, in the coated cermet tool, layers composing the hard-coating layer generally have a granular crystal structure, and the TiCN layer composing the titanium compound layer, the lower layer, is made to have a longitudinally grown crystal structure, which is formed by using a mixed gas that includes organic carbonitride as a reaction gas in a general chemical vapor deposition reactor at a moderate temperature range from 700 to 950° C. for the purpose of increasing the strength of the layer.
Furthermore, it is also well known that the α-Al2O3 layer composing the hard-coating layer of the coated cermet tool is formed with crystal grains having a corundum hexagonal close-packed structure, in which constituent atoms consisting of aluminum and oxygen exist in lattice points, that is, the atomic array in a unit lattice of the α-Al2O3 has a crystal structure shown in schematic views [FIG. 1A is a perspective view and FIG. 1B is a plan view showing cross-sectional layers 1 to 9].
[Patent Document 1] JP-A-6-31503
[Patent Document 2] JP-A-6-8010
In recent years, even though the performance of cutting devices has been markedly improved, labor saving, energy saving and cost reduction for cutting work are strongly demanded, and, accordingly, the cutting work is carried out at a higher speed range. The conventional coated cermet tools generally present no problem when they are used for continuous cutting or intermittent cutting of steel, cast iron or the like under normal conditions. However, when the conventional cutting tools are used for intermittent cutting under high-speed cutting conditions, the α-Al2O3 layer composing the hard-coating layer does not has sufficient impact resistance, thereby chipping (minute crack) easily occurs on the hard coating layer, and thus the lifespan of the cutting tool is reduced.